1. Field of the Invention
The present invention relates to an active matrix EL display device in which each pixel has TFTs (thin film transistors) and an EL element. Specifically, the invention relates to an active matrix EL display device of analog gray scale system in which gray scales are reflection of analog changes in amount of current flowing into EL elements.
In this specification, EL elements include those emit light from singlet excitation (fluorescence) and those emit light from triplet excitation (phosphorescence) both.
2. Description of the Related Art
With recent flood of data communication, demands for data communication equipment are increasing. In data communication equipment, display devices for displaying images are indispensable. The display devices that are attracting attention are EL display devices using an EL element that is a self-luminous element.
As display units larger in size and higher in definition are needed in data communication equipment, active matrix display devices in which each pixel has TFTs are becoming the main stream display devices.
FIG. 4 is a block diagram of an active matrix EL display device. A source signal line driving circuit 402 and a gate signal line driving circuit 403 are arranged in the periphery of a pixel portion 401. A signal outputted from the source signal line driving circuit 402 is inputted to source signal lines S1 to Sx to be sent to pixels. A signal outputted from the gate signal line driving circuit 403 is inputted to gate signal lines G1 to Gy to be sent to pixels. Power supply lines (power lines) V1 to Vx are arranged in parallel to the source signal lines to supply current to pixels.
As one way to reduce the size of a display device as well as manufacture cost, sometimes a pixel portion and a driving circuit portion (composed of a source signal line driving circuit and a gate signal line driving circuit) are formed on the same substrate. In this case, a polycrystalline semiconductor film is used to form TFTs that constitute the pixel portion and the driving circuit portion.
This can be applied to the active matrix EL display device of FIG. 4, and an example of the pixel structure thereof is shown in FIG. 5.
A switching TFT 504 has a gate electrode connected to a gate signal line G that is one of the gate signal lines G1 to Gy. The switching TFT also has a source region and a drain region one of which is connected to a source signal line S that is one of the source signal lines S1 to Sx and the other of which is connected to one of gate electrodes of a capacitor 505 and to a gate electrode of an EL driving TFT 506. Of two electrodes of the capacitor 505, one that is not connected to the switching TFT 504 is connected to a power supply line V that is one of the power supply lines V1 to Vx. The EL driving TFT 506 has a source region and a drain region one of which is connected to the power supply line V and the other of which is connected to an EL element 507.
In a pixel whose gate signal line G is selected, the signal electric potential of the source signal line S is inputted to one of the electrodes of the capacitor 505 through the switching TFT 504 that has been turned conductive. The voltage between the electrodes of the capacitor 505 is applied to the gate electrode of the EL driving TFT 506. In accordance with this voltage applied, a current flows from the power supply line V through the EL driving TFT 506 into the EL element 507 and causes the EL element 507 to emit light.
The luminance of light emitted from the EL element 507 is almost in proportion with the amount of current flowing into the EL element 507. Therefore gray scales are obtained by changing the amount of current flowing into the EL element 507.
In the display device shown in FIG. 5, the current flowing into the EL element 507 is inputted from the power supply line V through the EL driving TFT 506. The relation between a drain-source voltage VDS of a TFT and a drain current ID of the TFT in general is as shown in FIG. 8.
FIG. 8 is a graph showing plural ID curves obtained by varying the value of a gate voltage VGS. The drain current ID becomes larger as the absolute value of the difference between the gate voltage VGS and a threshold voltage Vth of the EL driving TFT 506 (|VGS−Vth|) becomes larger, in other words, as the absolute value VGS of the gate voltage VGS becomes larger.
When the absolute value |VGS−Vth| of the difference between the gate voltage VGS and the threshold voltage Vth of the EL driving TFT 506 is larger than the absolute value |VDS| of the drain-source voltage VDS, the TFT operates in a linear range. On the other hand, the TFT operates in a saturation range when |VGS−Vth| is equal to or smaller than the absolute value |VDS| of the drain-source voltage VDS.
The EL driving TFT 506 generally operates in the saturation range where the absolute value |VDS| of the drain-source voltage VDS is equal to or greater than the absolute value |VGS−Vth| of the difference between the gate voltage VGS and the threshold voltage Vth of the EL driving TFT 506.
In the saturation range, the drain current ID of the TFT is in proportion to the second power of the gate voltage VGS as shown in the following Equation 1.ID=(½)μ0C0(W2/L2)(VGS−Vth)2  (Equation 1)wherein, Vth represents the threshold voltage, μ0 represents the effective mobility, C0 represents the capacitance of a gate insulating film per unit area, W represents the gate width, and L represents the gate length.
In accordance with this equation, the electric potential to be inputted to the source signal line S is changed such that the TFT receives a gate voltage in proportion to the square root of the desired amount current to be inputted to the EL element 507. In this way, the EL element is caused to emit light of desired luminance.
When an image is to be displayed, an electric potential according to a desired gray scale is calculated by Equation 1 and is inputted to the source signal line.
However, a video signal inputted from the external generally has an analog electric potential that changes linearly with respect to the luminance obtained. Therefore accurate gray scales cannot be obtained when a video signal supplied from the external is inputted to the signal line as it is.
There is a countermeasure in which an external correction circuit converts the video signal into a drive signal in advance to suit the characteristics of the EL driving TFT and then the signal is sampled by the source signal line driving circuit and outputted to pixels to obtain a given gray scale.
This measure, however, complicates the operation since it requires video signal processing as above before the signal is inputted to the source signal line driving circuit. Furthermore, the measure needs the correction circuit in addition to the source signal line driving circuit to obstruct reduction in size of the display device.
Accordingly, a method has to be found which makes it possible to obtain a given gray scale when a video signal is inputted directly to the source signal line driving circuit.